Photovoltaic (PV) modules are made up of an outer glazing material, solar cells that are generally encapsulated in a clear packaging for protection, and a backsheet. The solar cells are made of materials known for use in solar collectors, including, but not limited to, silicon (crystalline and amorphous), cadmium indium selenide (CIS), cadmium indium gallium selenide (CIGS), and quantum dots. The back sheet is exposed to the environment on the backside of the photovoltaic module. The primary function of the back sheet is to provide electric insulation, low water vapor transmission, UV protection, and oxygen barrier properties necessary to protect the photocells (for example, silicon wafers) from degradation induced by reaction with water, oxygen and/or UV radiation. Because the photocells are generally encapsulated in ethylene vinyl acetate (EVA), or a thermoplastic encapsulant, the backsheet material should adhere well to EVA or the thermoplastic encapsulant when the components are laminated together.
Metal sheets, such as steel or aluminum have been used as backsheets in photovoltaic module. More recently polymeric materials have been used, such as TEDLAR, a polyvinyl fluoride (PVF) material from DuPont (U.S. Pat. No. 6,646,196), an ionomer/nylon alloy (U.S. Pat. No. 6,660,930), and polyethylene terephthalate (PET). PET exhibits excellent water vapor resistance at a relatively low cost; however, it is susceptible to degradation from exposure to environmental influences, such as UV and IR radiation, and ozone.
In many backsheet constructions, PET is protected by PVF films, which are tough, photo-stable, chemically resistant, unaffected by long-term moisture exposure, and adhere well to EVA after surface treatments. Typical constructions of photovoltaic back sheets are PVF/PET/PVF, PVF/Al/PVF and PVF/PET/Al/PVF multi-layered laminated films at 100 to 450 microns in thickness. Unfortunately PVF is a relatively expensive material. Further, PVF has relatively poor adhesion to PET. Adhesion is typically augmented by treatment of the polymeric surfaces to increase adhesion of the PVF film. An adhesive is needed for effective adhesion adding cost and manufacturing steps.
A polyvinylidene fluoride backsheet composition has been shown to provide performance, processing, and cost improvements over PET/PVF technology, as described in WO 08/157,159.
A thermoformable, high Tg methacrylic sheet has been proposed, especially for use in concentrating photovoltaic modules in U.S. 61/157,022.
There is a need for tough backsheet material that can be easily adhered to PET, and withstand the rigorous requirements of an effective backsheet, yet is less expensive than currently used materials.
Surprisingly, Applicant has found that an acrylic coating, film, or sheet can be easily adhered directly to PET, providing excellent weather and moisture protection at a lower cost than the currently used fluoropolymers. The PET is preferably pre-treated to improve adhesion. An additional advantage of the invention is that high levels of white pigment are easily incorporated into the acrylic material, providing excellent solar reflectance. Pigments are sometimes difficult to disperse compositions of presently used backsheet constructions, and therefore multiple layers are sometimes required for pigment incorporation, especially when large amounts of pigment are used. Such multiple-layer structures result in additional manufacturing steps and cost.